1. Field of the Invention
This invention relates to current limiting devices for general circuit protection including electrical distribution and motor control applications. In particular, the invention relates to current limiting devices that are capable of limiting the current in a circuit when a short circuit event or high current condition occurs, a conductive composite material used therein, and a method of manufacture of a conductive composite material.
2. Description of Related Art
There are numerous devices that are capable of limiting the current in a circuit when a high current condition occurs. One known limiting device includes a filled polymer material that exhibits what is commonly referred to as a PTCR (positive-temperature coefficient of resistance) or PTC effect. U.S. Pat. No. 5,382,938, U.S. Pat. No. 5,313,184, and European Published Patent Application No. 0,640,995 A1 each describes electrical devices relying on PTC behavior. The unique attribute of the PTCR or PTC effect is that at a certain switch temperature the PTCR material undergoes a transformation from a basically conductive material to a basically resistive material. In some of these prior current limiting devices, the PTCR material (typically polyethylene loaded with carbon black) is placed between pressure contact electrodes.
U.S. Pat. No. 5,614,881, to Duggal et al., issued Mar. 25, 1997, the entire contents of which are herein incorporated by reference, discloses a current limiting device. This current limiting device relies on a composite material and an inhomogeneous distribution of resistance structure.
Current limiting devices are used in many applications to protect sensitive components in an electrical circuit from high fault currents. Applications range from low voltage and low current electrical circuits to high voltage and high current electrical distribution systems. An important requirement for many applications is a fast current limiting response time, alternatively known as switching time, to minimize the peak fault current that develops.
In operation, current limiting devices are placed in a circuit to be protected. Under normal circuit conditions, the current limiting device is in a highly conducting state. When a high current condition, such as a short circuit, occurs, the PTCR material heats up through resistive heating until the temperature is above the xe2x80x9cswitch temperature.xe2x80x9d At this point, the PTCR material resistance changes to a high resistance state and the high current condition current is limited. When the high current condition is cleared, the current limiting device cools down over a time period, which may be a long time period, to below the switch temperature and returns to the highly conducting state. In the highly conducting state, the current limiting device is again capable of switching to the high resistance state in response to future high current condition events.
Known current limiting devices comprise electrodes and an electrically conductive composite material, which comprises a low pyrolysis or vaporization temperature polymeric binder matrix and an electrically conductive filler, combined with an inhomogeneous distribution of resistance structure. The switching action of these current limiting devices occurs when joule heating of the electrically conductive filler in the relatively higher resistance part of the composite material causes sufficient heating to cause pyrolysis or vaporization of the binder matrix, where at least one of material ablation and arcing occur at localized switching regions in the inhomogeneous distribution of resistance structure.
In order to attain specific and desired current limiting device properties in a reusable current limiting device, it has been proposed to control at least the concentration, morphology, and state of aggregation of the conductive filler material within the polymer matrix. This control may be accomplished using thermosetting polymers, where the conductive filler material is mixed with monomers, which that can be subsequently polymerized.
However, thermosetting polymers are often brittle. Thus, thermosetting monomers will not withstand a switching event or high current event without catastrophically fracturing, which of course is undesirable. Additionally, thermosetting polymers undergo substantial shrinkage during cure that can alter the microstructure of the material. Accordingly, for some applications it is not desirable to use a thermosetting polymer to control at least the concentration, morphology, and state of aggregation of the conductive filler material within the polymer matrix in a current limiting device.
Accordingly, it is desirable to provide a quick, reusable current limiting device, where the current limiting device overcomes the above noted, and other, disadvantages of the related art.
Further, it is desirable to provide an electrically conductive composite material and a method of manufacture of the electrically conductive composite material, for use in a quick, reusable current limiting device, where the current limiting device overcomes the above noted, and other, disadvantages of the related art.
Accordingly, it is desirable to provide a current limiting device comprising at least two electrodes; an electrically conducting composite material between the electrodes; interfaces between the electrodes and electrically conducting composite material; an inhomogeneous distribution of resistance at the interfaces so that during a high current event, adiabatic resistive heating at the interfaces causes rapid thermal expansion and vaporization and at least a partial physical separation at the interfaces; and means for exerting compressive pressure on the electrically conducting composite material. The electrically conducting composite material comprises at least one polymer matrix and at least one conductive filler. The at least one polymer matrix comprises at least one thermoplastic polymerized from cyclic oligomer.
Further, it is desirable to provide a method for forming a current limiting device with an electrically conducting composite material comprising at least one polymer matrix and at least one conductive filler, where the at least one polymer matrix comprises at least one thermoplastic polymerized from cyclic oligomer.
It is also desirable to provide a method for forming an electrically conducting composite material comprising at least one polymer matrix and at least one conductive filler, where the electrically conducting composite material is useable in a current limiting device and where the at least one polymer matrix comprises at least one thermoplastic polymerized from cyclic oligomers.
These and other advantages and salient features of the invention will become apparent from the following detailed description, which, when taken in conjunction with the annexed drawings, disclose preferred embodiments of the invention.